It is a conventionally known method that a polymer is produced through ring-opening polymerization of a ring-opening polymerizable monomer. For example, there is disclosed a method for producing polylactic acid by allowing a polymerization raw material containing lactide as a main component to react in a melted state to proceed to polymerization (see PTL 1). In accordance with the disclosed method, lactide is reacted to polymerize using tin as a catalyst, and setting a reaction temperature to 195° C.
When polylactic acid is produced by this production method, however, a polymer product contains more than 2% by weight of lactide residues (see PTL 1). This is because an equilibrium relationship between a ring-opening polymerizable monomer and a polymer is established in a reaction system of ring-opening polymerization of lactide, and a ring-opening polymerizable monomer tends to be generated by a depolymerization reaction when ring-opening polymerization of a ring-opening polymerizable monomer is performed at high temperature as the aforementioned reaction temperature. The lactide residues (ring-opening polymerizable monomer) may function as a catalyst for hydrolysis of a polymer product, or impair thermal resistance of the polymer product.
As for a method for carrying out ring-opening polymerization of a ring-opening polymerizable monomer at low temperature, there is disclosed a polymerization method using supercritical carbon dioxide as a solvent, and using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a catalyst (see NPL 1). In the disclosed method, lactide is polymerized by, after charging an autoclave with lactide, DBU, and benzyl alcohol, adding carbon dioxide thereto, mixing the mixture at the temperature of 80° C. and the pressure of 70 atm., and further adding carbon dioxide, followed by increasing the pressure to 250 atm. In accordance with this method, a polymer having a number average molecular weight of appropriately 10,000 is obtained by reacting for 16 hours.